I suggest you ...

Even after performing a transfer for a subscription, though it is visible at https://account.windowsazure.com/Subscriptions, the included Resource Group, Database Server, and SQL Database that belong to it are not seen at https://portal.azure.com/#blade/HubsExtension/Resources/resourceType/Microsoft.Resources%2Fresources until you go and access the “Access control (IAM)” screen and individually add the new transfer user as both an Owner and as a Co-administrator for the Database Server (under All Resources). I suggest all this be automated! I thought these settings would have been taken care of (or inherited) automatically when we initially transferred the subscription, but I was wrong. We were down unnecessarily for a little while until we figured this out (unnecessarily because had the documentation been more clear or adamant that these steps take place we would have caught the issue earlier, and unnecessarily again because in fact I am beginning to think all this was only administrative virtual paperwork that really didn't impact the operational status of the database while we got it all sorted out--so we probably could have continued using it anyway). Immediately after making those changes, the missing resource group, database server and the database showed up in the transfer user's Azure Portal! Additionally, I had to manually change the SERVICE ADMINISTRATOR e-mail separately on the Subscriptions page. This was also not set automatically by the subscription transfer routine. These are important additional steps that are easy to miss on Microsoft’s write-up at https://docs.microsoft.com/en-us/azure/billing/billing-subscription-transfer because that page only hints at it. I don't see why these steps couldn't all be lumped into the subscription transfer process either as a wizard UI or in an automated batch process. FYI - I’ve documented my exploits at https://dba.stackexchange.com/questions/216708/azure-sql-server-database-error-18456-severity-14-state-1/216711#216711.